Types of microphones include a dynamic type and a condenser type. The conventional dynamic microphone, which is consisted of a coil, a diaphragm and a permanent magnetic, operates based on the principle that a moving coil in the magnetic field generates electric current. The condenser type microphone, which mainly comprises two capacitor plates, a diaphragm and a black plate, operates under the working principle that sound pressure induces deformation of the diaphragm and changes the capacitance between the diaphragm and the back plate, such variation of the capacitance is converted into electric signals to be output.
The MEMS microphones is one of the most successful MEMS devices so far. The MEMS microphones is manufactured by surface processing or bulk silicon processing, which is compatible with the IC manufacturing. With the continuous device scaling down made in the CMOS technology, the MEMS microphone can be manufactured with an extremely small size, which enables its widely application in portable devices like cell phones, notebook computers, tablet computers or video cameras.
The MEMS microphone is generally a condenser type microphone having a diaphragm (lower electrode) fixedly formed on a substrate, opposed to a cavity in the substrate, and a back plate (upper electrode) suspended above the diaphragm with an air gap formed therebetween. However, such arrangement has the defect that time for releasing the sacrificial dielectric between the diaphragm and the back plate for forming the air gap need to be strictly controlled, since long releasing time may cause completely removal of the sacrificial layer and result in the peel off of the diaphragm. Furthermore, acoustic holes formed in the back plate communicating with the air gap will expose portion of the upper electrode, which makes the exposed upper electrode vulnerable to strong releasing solution.